Lithium ion batteries having high energy density and high voltage are commercially available and widely used as electrical power for consumer electronics, medical, military and vehicles. Such batteries generally use a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material. However, graphite-based anode materials have a theoretical capacity of only 372 mAh/g, thus suffering from limited capacity. Consequently, such graphite-based anodes are incapable of carrying out a sufficient role as an energy source for next-generation mobile equipment undergoing rapid development and advancement. Further, lithium metals, studied for use as the anode material, have a very high energy density and thus may realize a high capacity, but raise problems associated with safety concerns due to growth of dendrites and a shortened cycle life as charge/discharge cycles are repeated. Use of alternative, carbon-based electrodes, such as carbon nanotubes (CNTs), has been attempted as an anode active material, but various problems have been pointed out such as low productivity, expensiveness and low initial efficiency of less than 50%.
A number of studies have been recently proposed as to silicon, tin or alloys thereof, as they are known to be capable of performing reversible absorption (intercalation) and desorption (deintercalation) of large amounts of lithium ions through the chemical reaction with lithium. For example, silicon (Si) has a maximum theoretical capacity of about 4020 mAh/g (9800 mAh/cc, a specific gravity of 2.23), which is substantially greater than the graphite-based materials, and thereby is promising as a high-capacity anode material. However, when such silicon or tin materials are used as electrodes with conventional binders such as, for example polyvinylidene fluoride (PVdF) or styrene butadiene rubber (SBR), upon performing charge/discharge processes, the silicon and tin materials react with lithium. As a result of the reaction, the electrode undergoes significant changes in volume that can range from 200% to 300%. Due to such volume changes, repeated charge/discharge may result in separation of the anode active material from the current collector, or significant physicochemical changes at contact interfaces between the anode active materials, which are accompanied by increased resistance. As charge/discharge cycles are repeated, the battery capacity drops significantly, thereby shortening the cycle-life of the battery.
Increasing the amount of the binder can mitigate the volume changes can decrease separation of the material from the current collector, however, the electrical resistance of the electrode is increases. This leads to complications due to reduced battery capacity.
One suggested solution is use a polyvinyl alcohol or thermosetting plasticized polyvinyl alcohol having good adhesive strength as the binder. See JP Patent Publication Nos. 1999-67216, 2003-109596 and 2004-134208. However, such binders exhibit low viscosity, non-uniform application of the binder on copper foil as a current collector and processing problems associated with thermal treatment necessary to improve adhesion between the electrode mix and the current collector. WO 2000-007253 discloses a binder which includes a combination of polyvinyl alcohol and polyurethane. Korean Patent Application Publication No. 2006-001719 discloses a method of preparing an anode active material for a lithium secondary battery, using a resin composite which is prepared by coating graphite and at least one of Si, Sn and Al with a fixing agent such as a polyvinyl alcohol resin, a urethane resin, or the like. In connection with the preparation of the cathode for a lithium-sulfur secondary battery, Korean Patent Application Publication No. 2003-0032364 discloses a technique which involves preparation of an active material including a conductive material by binding between the sulfur active material and the conductive material using a combination of polymers, including polyvinyl alcohol and polyurethane as a first binder, and employs a second binder component insoluble in the solvent of the first binder, as a binder between the active materials including the conductive material and between the active material and the current collector.
Korean Patent Application Publication No. 2002-062193 discloses a semi-interpenetrating polymer network (semi-IPN) formed by the combination of a polyvinyl alcohol derivative and a compound having crosslinkable functional group(s), as a binder, wherein the polyvinyl alcohol derivative is a polymer compound having an oxyalkylene chain-containing polyvinyl alcohol unit in which hydroxyl groups are partially or completely substituted by oxyalkylene-containing groups, and compounds having two or more reactive double bonds are exemplified as the compound having crosslinkable functional group(s). However, partial or complete substitution of hydroxyl groups of polyvinyl alcohol leads to significant deterioration in physical properties of the polyvinyl alcohol, thus failing to express a desired level of adhesive strength.